1. Field of the Invention
This invention generally relates to a direct type backlight unit, and more particularly to a direct type backlight unit having a liftable frame structure.
2. Description of the Related Art
In general, a liquid crystal display typically has a backlight unit for providing light needed by its liquid crystal panel. However, the liquid crystal display may use different kinds of backlight units depending on its use and size. The backlight units mainly include two types, i.e. an edge type and a direct type.
Due to the advantages of high luminance, high light efficiency and simple structure, the direct type backlight unit is widely used in large-size liquid crystal display devices.
FIG. 1 shows an exploded view of a conventional liquid crystal display device 100. The liquid crystal display device 100 comprises a backlight unit 102, a set of optical films 104, a liquid crystal panel 106 and an outer frame 108. The backlight unit 102 is disposed under the liquid crystal panel 106 for distributing the light from a light source uniformly over the surface of the liquid crystal panel 106. The backlight unit 102 comprises a housing 103, a reflector 107 disposed upon the base surface of the housing 103, and lamps 110, such as cold cathode fluorescent lamps (CCFL), disposed within the housing 103. The set of optical films 104 is disposed between the backlight unit 102 and the liquid crystal panel 106, and includes a diffuser 104a disposed upon the backlight unit 102 and a plurality of optical sheets, such as a prism sheet 140b and a diffusing sheet 140c, disposed on the diffuser 104a. The outer frame 108 is disposed upon the liquid crystal panel 106 and the set of optical films 104 and connected to the backlight unit 102 so as to fix the liquid crystal panel 106 and the set of optical films 104 on the backlight unit 102, the outer frame 108 is fixed to the housing 103 by the screws. FIG. 2 shows a cross-sectional view of the assembled liquid crystal display 100 taken along line A-A in FIG. 1.
The lamps 110 are separately disposed within the cavity 105 formed by the housing 103 and positioned under the display area of the liquid crystal panel 106 so as to distribute the light uniformly over the surface of the liquid crystal panel 106. Each lamp 110 has two terminals 110a and 110b, and each terminal has a conductive end extending from the internal electrode of the lamp 110 and is connected to a conductive line 112 by a solder 114 formed by soldering as shown in FIG. 3; wherein the conductive line 112 is electrically connected to a power (not shown) so as to light up each lamp 110.
However, in the structure of the conventional liquid crystal display device 100, when the lamps are required to be repaired or replaced, the outer frame 108 should be first removed and then followed by the removal of the liquid crystal panel 106, the plurality of optical sheets 104c, 104d and the diffuser 104a from the backlight unit 102. Therefore, these removal steps might cause a difficulty in disassembling or assembling work.
In prior art, a backlight unit having a middle frame structure is provided for solving the above problem, which can be found, for example, in U.S. Pat. No. 5,432,626 issued to Sasuga et al. on Jul. 11, 1995, which is incorporated herein by reference. It discloses a liquid crystal display device having an outer frame, a middle frame and a lower frame wherein the middle frame is used for supporting a diffuser and a liquid crystal panel and fixed on the lower frame by a plurality of screws. Also, the outer frame has a plurality of fasteners for respectively connecting the middle frame and the lower frame. However, the connection structure of the liquid crystal display device disclosed by Sasuga et al. is accomplished by the plurality of screws and fasteners, which is also inconvenient for disassembling or assembling work.
Further, in prior art (including Sasuga et al. patent), the lamps 110 are electrically connected to a power by soldering as shown in FIG. 3. Therefore, the solder 114 is required to be removed before the lamp 110 will be taken out for repair or replacement work. Furthermore, the soldering manner has tedious processing steps and lower reliability. Also, if a defective solder is formed by a bad soldering process, the solder connecting the conductive end and the conductive line may be melted by a high temperature generated while the lamps are lit up and therefore results in short circuit. In addition, if a solder connecting the conductive end and the conductive line is not well isolated, it may generate point discharge and thus causes scorch or leakage problems.
Additionally, in prior art, the backlight unit needs to be reduced in thickness in order to make a thinner liquid crystal display device. However, if the distance between the liquid crystal panel and the lamps is too close, there will exist uneven luminance problem, i.e., bright lines and dark lines presented on the liquid crystal panel. Also, if the distance between the liquid crystal panel and the lamps is increased, the whole thickness of the liquid crystal display device will become larger. Further, if the distance between the liquid crystal panel and the lamps is too far, the luminance of the liquid crystal panel might be decreased to result in inefficient light utilization. Accordingly, the present invention provides a direct type backlight unit having a liftable frame structure so as to facilitate the assembling and disassembling works while replacing or repairing the lamps. Further, the present invention can reduce the backlight unit in thickness and avoid the uneven luminance problem at the same time.